Crash when selecting GPU or clicking System in settings with RX5700XT
In the event that it helps someone else out, I think the issue was an incompatibility with a kernel update. It's unfortunate but it appears that AMDGPU lags behind Ubuntu in kernel compatibility.

The machine recently updated to Kernel version 5.3. reverted it to 5.0 which was the previous version, reinstalled amdgpu and everything is fine again.
I installed amdgpu (pro version this time) 19.50, which is the latest version as of Jan 2020.